Most communication systems in widespread use today evolved from standards and equipment developed many years ago. This is especially true in the field of aircraft communications where equipment fielded one or two decades ago is still in use. Although these systems could be greatly enhanced using today's advances in communication technology, the cost of simply replacing them would be exorbitant due to the large quantity of this legacy equipment still in use.
However, newly manufactured equipment can be enhanced using advances in communication technology if these enhancements can be implemented in a manner that does not degrade the operation of current legacy equipment. It is acceptable that legacy equipment not be able to use or benefit from these enhancements as long as the enhancements do not degrade the performance of the legacy equipment.
One such ATC legacy system is the Mode S transponder whose reply data format is implemented using pulse position modulation (PPM). PPM is a modulation technique in which each bit interval is divided into two sub-intervals. A pulse is transmitted in one of the sub-intervals but not in both. Transmitting the pulse in the first sub-interval represents a “1” bit and represents a “0” bit when transmitted in the second subinterval.
PPM was initially chosen because it is very simple to both modulate and demodulate. It is a non-coherent modulation technique which ignores the phase of the modulation pulse. An opportunity therefore exists to enhance new Mode S equipment by phase modulating the PPM pulses.
Such a PPM pulse phase modulation technique is described in U.S. Pat. No. 8,031,105 “Systems and Methods for Enhanced ATC Overlay Modulation”. This patent describes an empirical based approach of using a second modulator, following the current PPM modulator, to phase modulate individual amplitude modulated output pulses of the first modulator with phase information.
Although this technique enhances the current Mode S transponder reply signal by converting the existing amplitude modulated pulses to Phase Shift Keying (PSK) modulated pulses, the technique is limited by the Mode S spectrum and pulse shape requirements. Careful adherence to pulse rise times and PSK modulation rate must be maintained to avoid violating Mode S signal specifications. Many of these phase modulation tradeoffs require empirical simulation and testing to ensure compliance which limits the benefits obtainable using this PPM remodulation technique.
Accordingly, it is the object of the present invention to disclose methods and apparatus which provide new and improved techniques for enhancing legacy communication systems without interfering with installed legacy equipment.